1. Field of the Invention
The present invention relates to a liquid crystal display device and a method for producing the same.
2. Description of the Related Art
Liquid crystal display devices (LCD) are in wide use as display devices for computers or television sets. So far, horizontal alignment type LCDs have been prevalent. A horizontal alignment type LCD operates in a liquid crystal display mode such as TN (Twisted Nematic) mode or STN (Super Twisted Nematic) mode using a positive nematic liquid crystal.
Recently, in order to provide improved viewing angle characteristics and display contrast, vertical alignment type LCDs using VAN (Vertical Aligned Nematic) mode have come into practical use. A vertical alignment type LCD is an LCD which performs display in a normally black (NB) mode by employing a vertical alignment type liquid crystal layer provided between a pair of electrodes.
In order to enhance the display contrast of a vertical alignment type LCD, it is necessary to control the alignment of the vertical alignment type liquid crystal layer so as to become more uniform, with an increased stringency.
One method of achieving alignment control of a liquid crystal layer is a method which ensures that the liquid crystal layer has a pretilt with no voltage applied across the liquid crystal layer. For example, in a TN type liquid crystal display device (which is a liquid crystal display device of a horizontal alignment type), the alignment control of the liquid crystal has conventionally been realized by controlling the pretilt (or more specifically, a pretilt angle and a pretilt direction) of liquid crystal molecules by using a horizontal alignment film which have been subjected to a rubbing treatment. The pretilt angle is determined by the material of the liquid crystal layer and the alignment films and the like, whereas the pretilt direction is determined by the rubbing direction. In such a liquid crystal display device, in the absence of an applied voltage, the liquid crystal molecules (liquid crystal directors) on the surface of the alignment films on the liquid crystal layer are not completely parallel to the substrates, but are inclined by about 1° to 6° (“pretilt angle”) in a predetermined direction (“pretilt direction”). Therefore, upon the application of a voltage across the liquid crystal layer, the liquid crystal molecules try to rise in the pretilt direction, thus causing a uniform and smooth change in optical response.
However, in the case of a vertical alignment type liquid crystal display device, the pretilt direction of the liquid crystal layer cannot be stably controlled even by performing a rubbing treatment for the vertical alignment films which are provided for the sake of alignment control. Moreover, since a vertical alignment type liquid crystal display device has a higher contrast than that of the horizontal alignment type liquid crystal display device, even a slight non-uniformity in alignment can be visually recognized, thus resulting in display unevenness.
Therefore, various methods of alignment control for a vertical alignment type liquid crystal display device have been studied. For example, there have been proposed a method of providing protrusions within the pixels (“rib technique”) and a method of providing slits in the electrodes (“fringe field technique”). According to these methods, without having to perform a rubbing treatment for the alignment films, it is possible to restrict liquid crystal orientations by means of the rib structures or fringe field (i.e., inclined electric field).
By using the rib technique or the fringe field technique, not only is it possible to realize more stable alignment control than in the case of a rubbing treatment, but an advantage also exists in that alignment division is relatively facilitated (MVA mode; Multi Domain Vertical Alignment). In MVA mode, a plurality of regions (“domains”) having different orientation directions (e.g., pretilt directions) are allowed to exist within each pixel, while ensuring that the areas of such domains are averaged out. Thus, it is possible to reduce drastic changes in brightness or contrast in response to changing viewing directions, whereby the viewing angle characteristics can be greatly improved.
As the simplest method for realizing alignment division, there has been disclosed a method which divides one pixel into four parts, as shown in FIG. 1 (e.g., Japanese Patent No. 2947350). Hereinafter, alignment division will be described by taking the method shown in FIG. 1 as an example.
Under no applied voltage, as shown in FIG. 2A, liquid crystal molecules 12 (hereinafter referred to as “central molecules”) located at a middle level along the direction of the liquid crystal layer in each of the four split regions (“domain”) are oriented in a direction substantially perpendicular to the face of each substrate 11 on which a vertical alignment film is formed. Provided that a pair of polarizers 11 are disposed so that their transmission axes lie perpendicular to each other (cross Nicol) with the liquid crystal layer interposed therebetween, light is not transmitted through the liquid crystal layer, thus resulting in a “black” display state.
Next, when a voltage is applied across the liquid crystal layer, as shown in FIG. 2B, the central molecules 12 fall in a direction as restricted by the ribs or fringe fields. As a result, light is now transmitted through the liquid crystal layer due to the birefringence thereof. If each pixel is alignment-divided so that, as shown in FIG. 1, the direction in which the central molecules 12 fall in each domain (as indicated by an arrow 13) differs from domain to domain, excellent viewing angle characteristics can be obtained as long as the areas of the four domains are averaged out, despite the less-than-optimum viewing angle characteristics of each domain.
If the above-described alignment division were to be realized without providing ribs or slits in each pixel, it would be necessary to form vertical alignment films which can create a plurality of domains having different pretilt directions within a single pixel, for example. However, according to any conventional technique which employs a rubbing treatment, rubbing would have to be performed a plurality of times (e.g., four times) in different directions, each time for a different domain. Since cloth is to be used for rubbing, the division precision would become poor, thus making practical applications difficult.
On the other hand, the rib technique and the fringe field technique also have a problem in that, since ribs or slits are provided within each pixel, the aperture ratio decreases, thus resulting in a dark display. As used herein, an aperture ratio is a ratio in area, to one pixel, of a portion of the pixel that allows light to be transmitted therethrough. Furthermore, the structures of the substrates, electrodes, and like elements may become complicated, so that the productivity may be lowered and the production cost may increase with increase in the number of steps involved in the production process.
Therefore, one method which is currently under study is, without using a rubbing treatment, forming vertical alignment films having a predetermined surface configuration, and controlling the pretilt direction of a vertical alignment type liquid crystal layer by utilizing the surface configuration of such vertical alignment films. Proposals have been directed to a method which forms periodic undulations (ruggednesses) with a minute pitch on the surface of each vertical alignment film, and a method which provides a vertical alignment film on a base film having a predetermined surface configuration to control the surface configuration of each vertical alignment film.
For example, a method has been proposed in which a vertical alignment film is applied to a substrate on whose surface an SiO film is formed by oblique evaporation (see, for example, T. UCHIDA, M. OHGAWARA, M. WADA, Jpn. J. Appl. Phys., 19, pp. 2127-2136 (1980)). An SiO film which is obtained by oblique evaporation has a surface configuration characterized by an arrangement of minute columns (unit features). According to the method of UCHIDA et al., the pretilt direction is controlled by the surface configuration of the SiO film. UCHIDA et al. also describe that the pretilt angle can be controlled through adjustment of the surface configuration of the SiO film by varying the evaporation conditions.
In Japanese Laid-Open Patent Publication No. 3-150530, there is proposed a method which performs embossing on the surface of a vertical alignment film by using, as a pressing die, a glass substrate having grooves in the shape of a diffraction grating or a substrate on whose surface SiO is obliquely vapor deposited.
The method proposed in UCHIDA et al. and the method proposed in Japanese Laid-Open Patent Publication No. 3-150530, supra, are both directed to producing a structure such as a substrate or a pressing die having a predetermined surface configuration, and forming a vertical alignment film having a surface configuration which reflects the surface configuration of that structure. However, these methods have the following problems because oblique evaporation is utilized for producing such a structure.
Firstly, it is difficult with oblique evaporation to control the surface configuration of a structure to a high precision. This problem is particularly outstanding in the case where unit features are to be formed on a vertical alignment film surface with a small pitch of, e.g., several μm or less. Secondly, it is impossible to arbitrarily prescribe the configuration of each unit feature of the structure (i.e., angle, orientation, etc., of the slanted faces of the grooves). Since the configuration of unit features which are formed on the surface of an SiO film by oblique evaporation depends on the evaporation conditions, there are limits to the configuration of the unit features that can be selected. Therefore, it is difficult to obtain a pretilt with an arbitrary direction or angle, and thus, there are limitations on the applications of the display device. Thirdly, in the case where an alignment division is to be performed for improved viewing angle characteristics (MVA mode), it is necessary to form a vertical alignment film which permits a plurality of regions (domains) having different pretilt directions to exist within one pixel. Using oblique evaporation to produce a structure for forming such a vertical alignment film, however, would complicate the production process. Moreover, with any method utilizing oblique evaporation, it is necessary to secure a certain distance or more between the evaporation source and the substrate surface in order to ensure that the incident angle with respect to the substrate surface falls within a predetermined range. Thus, pompous equipment is required, thus making the production of large-sized display devices difficult.
On the other hand, in Y. KAWAI, I. IRIE, T. SHIMAMURA, T. KAGASHIRO, H. OKADA, and H. ONNAGAWA, “Control of nematic liquid crystal alignment using an ultra-fine periodical structures”, preprints of 2002 liquid crystal symposium, pp. 111-112, there is proposed a method which forms ruggednesses composed of periodic fine grooves on a substrate surface by utilizing interference exposure, thus causing vertical alignment of liquid crystal.
However, KAWAI et al. lack any mention of causing a pretilt of vertically aligned liquid crystal molecules. Moreover, the ruggednesses which are described KAWAI et al. are obtained by allowing perpendicularly-intersecting sinusoidal interference fringes to exist, and therefore, there are limitations on the configuration and arrangement of the fine grooves that can be selected. Furthermore, since similar features are formed along two directions perpendicular to each other (x direction, y direction), it is difficult to separately control the features along the x direction from the features along the y direction. Therefore, when this method is applied to a display device of MVA mode, for example, the production process may be complicated.
As described above, although there have been proposed methods for providing minute undulations (ruggednesses) on a surface which is in contact with a liquid crystal layer in order to perform alignment control of a vertical alignment type liquid crystal layer, it is difficult to obtain arbitrary and strict control of liquid crystal alignment without lowering the aperture ratio or complicating the production process.